1. Field of the Invention
The present invention relates to a slurry managing system and a slurry managing method for wire saws that cut workpieces, such as silicon ingots, into thin plates by using slurry that includes abrasive grains. More particularly, the present invention pertains to a slurry managing system and a slurry managing method that is capable of controlling the percentage content of abrasive grains contained in the slurry at an optimum level.
2. Description of the Related Art
A so-called inner saw, which is a cutting apparatus, was employed in the prior art to produce wafers from materials such as ceramic, substrates for IC chips, silicon for solar batterys, and synthetic quartz. This apparatus uses a cutting blade to cut the material and produce wafers one at a time. Therefore, it takes a long time to produce a large number of wafers. In addition, the cutting blade produces cutting chips and consumes a large volume of the material. This decreases the number of wafers that may be produced from a single workpiece.
To solve these problems, a so-called wire saw has been developed in recent years. As shown in FIG. 19, a typical wire saw includes three rollers 12a, 12b, 12c, a wire 10, and supply pipes 13. The wire 10 is spirally wound about the rollers 12a, 12b, 12c with a predetermined interval between each winding. The supply pipes 13 supply slurry to the wire 10. The wire 10 extending between the upper rollers 12a, 12b constitutes a wire group 11 in which each segment of the wire 10 is parallel to an adjacent segment of the wire 10. The slurry includes a dispersing liquid, which is composed of either water or oil, and abrasive grains dispersed by the liquid. The wire saw further includes a receiving tank 14 located in a cutting zone and a slurry tank 15 located at the downstream side of the receiving tank 14. The slurry tank 15 contains slurry and has an agitator 19 arranged therein to agitate the slurry. A pipe 16, which includes a valve V1 and a pump P1, connects the supply pipes 13 to the slurry tank 15. The slurry tank 15 is provided with a slurry supply port 15a through which fresh slurry is provided. A discharge pipe 17, which includes a valve V2, is connected with the slurry tank 15.
To cut a cylindrical workpiece 18 with the above wire saw, the valve V2 is first closed and the valve V1 is opened. The slurry inside the slurry tank 15 is provided to the wire group 11 through the pipe 16 and the supply pipes 13 by the pump P1. Simultaneously, the rollers 12a, 12b, 12c are rotated in a clockwise direction, as viewed in FIG. 19, to feed the wire group 11 toward the right. In this state, the workpiece 18 arranged above the wire group 11 is lowered until it reaches a position below the wire group 11. As the workpiece 18 passes through the wire group 11, the workpiece 18 is sliced and a plurality of wafers are produced.
The cutting capability of the wire group 11 depends largely on the mixed weight ratio of the abrasive grains and the dispersing liquid in the slurry. Hence, for efficient cutting of the workpiece 18, it is necessary to maintain the mixed weight ratio of the abrasive grains and the dispersing liquid in the slurry at a value which brings the cutting capability of the wire group 11 to a maximum level.
During the above cutting process, the slurry provided to the wire group 11 through the supply pipes 13 are received by the receiving tank 14 and collected in the slurry tank 15. After the cutting process is carried out for a predetermined number of cycles, a certain amount of the used slurry is discharged from the slurry tank 15 and the same amount of fresh slurry is supplied to the slurry tank 15 through the supply port 15a. The slurry is regenerated in this manner.
Replacement of a portion of the slurry is carried out due to the cutting chips that become mixed with the slurry during the cutting of the workpiece 18. The amount of cutting chips that enter the slurry increases as the number of the completed cutting cycles increases. Furthermore, the abrasive grains in the slurry are fragmented during the cutting. This reduces the amount of abrasive grains that may be used for cutting. The cutting chips mixed with the slurry and the reduction of the usable abrasive grains lowers the cutting efficiency. Therefore, a portion of the slurry is replaced periodically to prevent this problem. However, the amount of cutting chips and fragmented abrasive grains increases gradually regardless of the periodic replacement of the slurry. Thus, the slurry is completely replaced with fresh slurry after the cutting process is carried out for, for example, a dozen times.
The slurry discharged from the slurry tank 15 during replacement of the slurry is disposed as slurry waste. The slurry waste includes abrasive grains and dispersing liquid that may still be used. The replacement of such slurry with fresh slurry increases the consumption of the abrasive grains and the dispersing liquid required for the cutting. As a result, this increases the cost of the process. In addition, dispersing liquid must be disposed as industrial waste, which adds to the costs.
During replacement of the slurry, fresh abrasive grains and dispersing liquid are provided in the slurry tank 15 and mixed to form slurry. Therefore, a certain length of time is required to replace the slurry. The cutting of the workpiece 18 must be stopped during the replacement since the amount of slurry becomes insufficient during replacement. Replacement of all the slurry takes more time than a partial replacement. The delays caused by slurry replacement lower productivity.